1. Field of the Invention
This invention relates to an ultrasound transmission imaging system or ultrasonic transmission camera. In particular this invention relates to the insonification apparatus or ultrasound transmitter of an ultrasound transmission system or camera. Still more particularly, this invention relates to the insonification apparatus of a C-scan ultrasound transmission system having an extended ultrasonic wave source. The insonification apparatus is preferably used for medical examinations of a patient.
2. Description of the Prior Art
An ultrasound transmission system incorporating an insonification apparatus of the type contemplated herein is described, for instance, in U.S. Pat. No. 3,937,066 and in the article by J. F. Havlice, P. S. Green, J. C. Taenzer and W. F. Mullen entitled "Spatially and Temporally Varying Insonification for the Elimination of Spurious Detail in Acoustic Transmission Imaging", published in Acoustical Holography, Vol. 7, Plenum Publishing Corrporation (1977), pp. 291-305.
In these prior art publications, real-time acoustic transmission imaging systems are described. Part of the attractiveness of transmission imaging lies in its orthographic image presentation. The images are readily interpretable. Since the images are formed by the attenuation properties of the tissues of a patient under examination, the images may contain diagnostically significant information.
In the aforementioned article in Acoustical Holography, supra, imaging with diffuse sound is suggested. An extended ultrasonic wave-generating transducer, in particular a multi-element transmitting array, is used to reduce spurious detail in the acoustic images produced by the imaging lens.
However, it has been found that the contrast or spatial resolution of the acoustic images of such an ultrasound transmission system can be improved even more. A measure of spatial resolution is the so-called "modulation transfer function", as is well-known in the imaging art. Therefore, there is the strong desire to improve the modulation transfer function of the system.
Due to the fact that in diffuse insonification sound waves impinge into the object plane from a large number of directions, shadowing effects due to out-of-plane objects are largely eliminated. In the prior art article in Holography, supra, it is expressly stated that each point of the object plane is insonified "from many directions". Yet, in order to improve the performance of the system, the object plane should receive ultrasound waves preferably from all directions. In other words, each point in the image should contain information from each and every transmitter element of the ultrasound wave-generating transducer. In fact, it has been found that in the prior art insonification system full advantage has not been taken of the wide angular acceptance angles of the imaging lens.
In addition, it has been found that the image field is not insonified uniformly (when no object is present). Thus, another problem of the prior art design resides in the fact that the image produced from the object plane shows a sharp decline of the intensity towards its edges. In other words, there is a predominance of intensity in the center of the image (in the absence of the object). Yet, it is desirable to have an image in which the intensity is equally distributed across the image.
Composite acoustic lens assemblies which are adapted for use in a fluid medium and utilized for forming acoustic images with incident acoustic waves coming from an object under examination are disclosed in U.S. Pat. No. 3,982,223.